With a pre-1801 POD, the orange-white-blue Prince's Flag might be a better basis than the modern red-white-blue. As such, I'd replace the upper band of blue field on the great union flag with orange. Basically it'd be
@Battlestar_Cydonia 's second design above, but with orange instead of red.
Like so:
It certainly works better IMHO than having the same red across the top as is used in the St George's Cross. Though while reading up on older Dutch flags I found that they actually changed orange to red far earlier than I'd thought. Certainly for the Dutch
navy, it seems that the Prince's Flag was the only one in use up to 1630-ish, then there was a period where it and the current national flag (State Flag a.k.a. Statenvlag) vied against each other, before the State Flag became universal in the 1660s. Dutch Wikipedia also claims the flag was changed in order to curry favour with Cromwell's republic in England, as the House of Stuart apparently also used orange as a house colour, just like the House of Orange-Nassau. But this doesn't tally with a change-over beginning in 1630, 18 years before the English Civil War began.
I suspect that the reason for the change is really far more prosaic. Certainly with the Union Flag, the Royal Navy tended to have theirs made using much darker dyes than the Army etc. preferred because their flags bleached out faster in the wet, salty, windy environment at sea. I suspect as the Dutch kept a similar eye on their finances that the same tendency arose there too, but starting from a much lighter base than the Union Flag does it rapidly diverged into two officially separate designs. The Prince's Flag also has a much lighter shade of blue than the State Flag, after all. Then following the Navy's change it also began to supplant the Prince's Flag on VOC ships and on land.
But I'm rambling a bit. It's still early in the morning for me, haven't woken up properly yet!